Derivatives (Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	$ 600	$ 1,508
Fair value, derivative liability	1,458	2,035
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	370	747
Fair value, derivative liability	1,406	2,035
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	230	761
Fair value, derivative liability	52	0
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Prepaid and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	239	702
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	45
Natural Gas Contracts | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	29	0
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Prepaid and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	230	732
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	0	29
Natural Gas Contracts | Not Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	52	0
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative asset	131	0
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Derivative Liability, Current [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	1,377	1,928
Interest Rate Swap [Member] | Designated as Hedging Instrument [Member] | Other Long-Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair value, derivative liability	$ 0	$ 107